Various types of imagers or image sensors are in use today, including charge-coupled device (CCD) image sensors and complementary metal-oxide semiconductor (CMOS) image sensors. These semiconductor-based image sensors are widely used in many image input devices because they can be mass produced using advanced fine-patterning lithographic techniques. Applications include digital cameras, computer peripherals for document capture, visual communications, and facsimile machines.
A CCD image sensor utilizes an array of photo sensors to form charge packets proportional to the received light intensity. These photo sensors are typically photo transistors or photo diodes located on the image sensor surface. Each charge packet constitutes a pixel of the composite image. The image data is read out from the CCD array by shifting these analog charge packets from the CCD array interior to the periphery in a pixel-by-pixel manner. To begin the readout process, the charges on the first row are transferred to a readout register and from there the signals are input to an amplifier and in most applications to an analog-to-digital converter. Once a row has been read, its charges on the readout register row are deleted. The next row then enters the readout register and all the rows above move down one row. In this way, each row is read, one row at a time. Because all the pixels in a row of pixels are read simultaneously, the pixels of the CCD array are not individually addressable.
Due to voltage, capacitance and process constraints, CCD arrays are not well suited to integration at the high levels of integration possible in CMOS integrated circuits. Hence, any supplemental signal processing circuitry required for the CCD image sensors (e.g., memory for storing information related to the sensor) is generally provided on one or more separate chips. As a result, the system cost and size are increased. It is also known that CCD image sensors require a large power consumption and higher operating voltages, as compared with conventional CMOS signal processing circuitry.
CMOS image sensors typically utilize an array of active pixel image sensors and a row or register of amplifiers to sample and hold the output of a given row of pixel image sensors. The principle of a CMOS pixel's operation is based on the modulation of a reverse biased an junction capacitance (of a diode, for example) due to impinging light. Photons absorbed in the depletion region of the reverse biased junction generate electron-hole pairs that discharge the reverse biased capacitance. Larger junctions collect more photons and are more sensitive to light, but larger junctions also reduce the resolution of a sensor because fewer pixels can be placed on the available surface area.
CMOS image sensors have several advantages over CCD image sensors. CMOS image sensors are formed with the same CMOS process technology used for the associated circuitry required to operate the CMOS image sensor and therefore the sensors and support circuitry are easily integratable into a single chip. Single chip integration eases miniaturization, lowers manufacturing costs, and boosts reliability. Using CMOS image sensors, it is possible to create a monolithic integrated circuit providing not only the sensor but also control logic and timing, image processing, and signal-processing circuitry. Thus the CMOS image sensors can be manufactured at lower cost, relative to CCD image sensors, using conventional CMOS integrated circuit fabrication processes. Also, the CMOS image sensors operate at a lower operating voltage and consume less power, allowing the system into which the sensors are incorporated to operate longer on batteries, which is a major advantage for hand-held imaging products. Finally, each CMOS image sensor is accessible over a grid of x-y lines, instead of using the shift register process of charged coupled devices. The column and row addressability of the CMOS image sensor, which is similar to the conventional RAM readout process, allows windowing of the image. CMOS image sensors require only a single power supply to drive both the image sensor and the associated circuitry. By contrast, CCD image sensors typically require three different input voltages. Also, CCD image sensors lack a consistent dark level voltage due to fabrication processing imperfections. CMOS image sensors are also known to exhibit inconsistent dark levels, but the associated CMOS signal processing circuitry can track the dark level for each CMOS image sensor and provide a compensation factor during the signal processing function so that a uniform dark level is achievable across the CMOS image sensor array.
However, CMOS image sensors are not without disadvantages. The use of state-of-the-art CMOS integrated circuit fabrication techniques for the associated signal processing circuitry, and the CMOS image sensor would compromise the construction of the CMOS photo sensors, thereby reducing the image signal quality. For example, typical substrate and source/drain doping levels (or retrograde doped tubs where the doping level at the surface is lower than the doping level below the surface) conventionally used in CMOS processes are higher than the doping levels that provide optimal image sensor quality. Reducing the doping levels to achieve better sensor sensitivity, dynamic range, or color balance, would significantly degrade the performance of the CMOS processing circuitry. Therefore, higher levels of component integration (i.e., image sensors and operative signal processing circuitry on the same chip) are therefore not practical.
Further, in those situations where the CMOS image sensor and its signal processing circuitry are co-located on the same integrated circuit, the associated circuits consume a portion of the available pixel area, resulting in a larger overall chip area and reducing the image fill factor (the ratio of the active pixel area to the total pixel area). The efficiency, resolution and sensitivity of the CMOS image sensor array is in turn disadvantageously reduced. Also, certain CMOS material layers (e.g., suicide layers) may be partially or completely opaque, reducing the image sensor sensitivity. In an effort to overcome the disadvantages created when using state-of the-art CMOS process technology in conjunction with CMOS image sensors, certain modified CMOS processes have been created that remove processing steps or alter device physical characteristics to improve the image sensor signal quality. Although removal of these process steps improves image sensor signal quality, the CMOS technology is generally compromised. In summary, it can be said that state-of-the-art CMOS image sensor processing technology lags by several generations the current state of the CMOS processing art.